For enterprise and support, Apple still hosts the binaries. Using softwareupdate --fetch-full-installer --full-installer-version 10.13.6 in Terminal on a Mac running older OS versions sometimes works.
Apple no longer lists High Sierra prominently, but you can still download it from Apple’s official support page: macos high sierra 10136 dmg high quality
If the output matches established community hashes, you have a high-quality copy. For enterprise and support, Apple still hosts the binaries
/Volumes/Install\ macOS\ High\ Sierra/Install\ macOS\ High\ Sierra.app/Contents/System/Library/CoreServices/SystemVersion.plist For enterprise and support
High Sierra is incredibly versatile. It runs on: